is, therefore, a limit to the tension of the vapour, which varies with the temperature, but which, for a given temperature, is independent of pressure. In order to show that, in a closed space saturated with vapour and containing liquid in excess, the temperature being constant, there is a maximum of tension which the vapour cannot pass, whatever may be the pressure, we employ a barometric tube immersed in a deep cistern, fig. 81, p. 277, vol. iv. This tube is first filled with mercury, and a quantity of ether is passed into the tube sufficient to saturate the barometric chamber; there is then some liquid in excess, and the height of the mercury in the tube is ascertained by means of a scale fixed to the cistern. Now, whether the tube be immersed to a greater depth, which tends to compress the vapour; or whether it be raised, which tends to expand it, the height of the mercurial column remains constant. The tension of the vapour, therefore, remains the same in both cases, since the depression neither increases nor diminishes it. Hence, it follows, that when the vapour contained in a saturated space is compressed, a part of it returns to the liquid state; and that if, on the contrary, the pressure is diminished, a portion of the liquid remaining in excess is vaporised, and the space occupied by the vapour is saturated anew; but in both cases, the tension and the density remain constant. If the space where the vapour is contained be not saturated, or if it do not contain liquid in excess, the vapour, when the pressure increases or diminishes, acts entirely as a gas; that is, so long as it is not brought up to the point of saturation, its tension and its density increase with the pressure. Consequently, it is evident that vapours, in a space not saturated, act according to the law of Mariotte. Tension of the Vapour of Water below the Freezing Point.-In order to measure the elastic force of the vapour of water below 0° Centigrade, Gay-Lussac employed two barometric tubes filled with mercury and immersed in the same cistern. One of these, completely freed from air and humidity, was used to measure the pressure of the atmosphere: into the other a small quantity of water was introduced, and its barometric chamber was surrounded with a small jacket, in which was placed a frigorific mixture. By comparing the heights of the two barometers when the temperature of the frigorific mixture stood at different points of the scale, Gay-Lussac found that in the barometer which contained the water, the depression of the mercury, and consequently the tension of the vapour, were as follows: TABLE OF THE ELASTIC FORCE OF THE VAPOUR OF WATER. Hence it is inferred that, at very low temperatures, there is still vapour of water in the air. Tension of the Vapour of Water from the Freezing to the Boiling Point.-We shall first give the process adopted by Mr. Dalton, of Manchester, who died in 1844, in order to determine the elastic force of the vapour of water from 0° to 100° Centigrade. He employed two barometric tubes A and B, fig. 188, which were immersed in an iron vessel full of mercury and placed over a furnace. The barometer B was freed from air and humidity, and in the barometer A was put a small quantity of water. These two barometers were kept in a vessel of glass full of water, in the middle of which was immersed a thermometer T, which indicated the temperature of the liquid. By gradually heating the iron vessel, and consequently the water in the glass vessel, that which was in the tube was vaporised; and in proportion as the tension of the vapour increased, the mercury was lowered. Then, by marking degree after degree on the scale E, the depression which took place in the tube a, below the level B in the other tube, Mr. Dalton determined the elastic force of the vapour of water at every point of the thermometer between the freezing and the boiling points, and was the first to construct a table of the same, as follows: ture, as given by these eminent experimenters, is the following :E=(1+007153 T) 5; in which E, denotes the elasticity of the steam in atmospheres; and T, the excess of the temperature above 100° Centigrade. The same formula adapted to Fahrenheit's thermometer is as follows: in which e denotes the elasticity of the steam in atmospheres; and t the temperature above 212° Fahrenheit. The following table is derived from the table given by MM. Dulong and Arago, in their Report, and it is extended from twenty-four to fifty-three atmospheres, by calculation, according to the preceding formula: Temperature. Fahrenheit. Tension of Steam above the Boiling Point.-Two methods have been employed in measuring the elastic force of steam at higher temperatures than 100 Centigrade, the one by MM. Dulong and Arago in 1830; the other by M. Regnault in 1844. The apparatus of the former experimenters consisted of a boiler of very thick iron-plate, capable of holding 80 litres, or about 178 imperial gallons. Two gun barrels, closed at their lower extremity, were immersed in the water of the boiler, to the sides of which they were firmly fastened. Each barrel was filled with mercury, and contained a thermometer intended to show the temperature of the water and of the steam in the interior of the boiler. In order to measure the tension of the steam, the boiler was put in communication with a manometer of compressed air, which had been experimentally graduated. By noting degree after degree the temperatures indicated by the thermometers, and observing at the same time the indications of the manometer, these experimenters actually measured the tension of steam up to twenty-four atmospheres. They then determined by means of the following formula, temperatures and the pressures of steam as far as fifty atmospheres. These researches having been made at the instance of the Royal Academy of Sciences of Paris, a report of them was published in the "Memoirs of the Academy," vol. x. 1831. The formula which connects the elasticity of steam with the tempera tion of the elasticity of steam above the boiling point, which The apparatus adopted by M. Regnault for the determinawe now proceed to describe, admits of the measurement of the tension of the vapour of water either above or below 100° Centigrade. It consists in boiling water in a close vessel, under known pressure, and in measuring the temperature at which ebullition is effected. Then on the principle that at the instant of ebullition the elastic force of the vapour or steam disengaged is exactly equal to the pressure which the liquid supports, we ascertain the tension of the steam or vapour and its corresponding temperature, which resolves the problem. The apparatus is composed of a brass vessel c, fig. 189, hermetically closed and filled with water to about one-third of its capacity. Four thermometers are inserted in the cover; two immersed in the upper strata of the liquid, and the other two in the lower strata. From the reservoir c, proceeds a tube A B, which is adapted to the orifice of a glass globe, having the capacity of twenty-four litres, or 5-28 imperial gallons, and filled with air. The tube AB is surrounded with a jacket D, in which circulates a current of cold water, which Hows from a reservoir E. From the upper part of the globe м proceed two tubes, the one communicating with a manometer of free air o, near the apparatus, and the other HH', made of lead, communicating with an air-pump, or with a forcing-pump, according as the air in the globe is to be rarefied or compressed; and the reservoir K, which contains the globe, is filled with water at the surrounding temperature. Suppose now that the first experiments are to measure the elastic force of the vapour of water below 100° Centigrade. The extremity H' of the leaden pipe is fixed to the platen of the air-pump, and the air in the globe м is rarefied, and conse ! quently that in the vessel c. Then, by heating this vessel FROM 100 TO 230°-9 CENT., ACCORDING TO M. REGNAULT. TEMPERATURE. Cent. 1000.0 120 .6 1234 TEMPERATURE, Pressure in Atmos. slowly, the water which it contains enters into a state of ebulli- TABLE OF THE ELASTICITY OF STEAM IN ATMOSPHERES, 133 .9 Pressure Fahr. 2120.00 Cent. 1980-8 Fabr. 389 84 15 249.08 201 9 273 .02 These tables show that the elastic force or pressure of the vapour of water and steam increases according to a certain law more rapidly than the temperature; but this law is not yet clearly ascertained. The table of M. Regnault differs from that of MM. Dulong and Arago; and, of course, the empirical formula given by these philosophers does not quite apply to the former; by calculation, this formula gives 27-22 atmospheres instead of 28, for the temperature of 230° 9 Centigrade or 447° 62 Fahrenheit. Water is the only liquid whose vapour, from its important applications, has engaged the attention of philosophers. The elastic force of the vapours of other liquids has not been determined with accuracy or to any extent. It is known, however, that substances in solution, as salts and acids, at the the same temperature as that of water, diminish the elastic force of the vapour of such mixtures, and this diminution increases in proportion as the solution becomes more concentrated; for ebullition then takes place only at a higher temperature. The following table will show the boiling points of water mixed with salt in certain proportions up to the point of saturation. A of this liquid be the same in both tubes. The globe and its stop-cock are now removed, and in their place is put the funnel c, furnished with a stop-cock a, which differs in its construction from the ordinary stop-cocks. It is not pierced through and through, but has only a small cavity as seen at o on the right of the figure. Having poured into the funnel o the liquid which is to be vaporised, having marked the level I of the mercury and opened the stop-cock b, the stop-cock a is then turned in such a manner that the cavity is filled with the liquid; it is then turned again, so that the liquid may enter the space A, and there be vaporised. Thus the liquid is made to enter, drop by drop, until the air in the tube is saturated with vapour, which is ascertained by the level ceasing to descend. As the tension of the vapour produced in the space A is added to that of the air already there, the volume is increased; but it is easily reduced to its original volume, by pouring an additional quantity of mercury into the tube B. When the mercury is by this means made to rise in the large tube to the level 1 which it had at first, there is observed in the tubes B and A a difference of level Bo, which evidently represents the tension of the vapour which has been produced; for the air having resumed its original volume, its tension is not changed. Now if some drops of the same liquid which was introduced into the space A be passed into the barometric vacuum, a depression exactly equal to Bo is observed; and this proves clearly that, at the same temperature, the tension of a vapour is the same in a gas as in a vacuum. As to the second law, it is proved by the preceding experiment, because when the mercury has resumed its level 1, the mixture supports the atmospheric pressure which acts at the top of the tube F, plus the weight of the column of mercury BO. Now these two pressures exactly represent, the one the tension of the dry air, and the other the tension of the vapour. Whence, the second law may be considered as a consequence of the first. The apparatus which we have described only admits of experiments at the ordinary temperature; but M. Regnault, by means of an apparatus capable of being employed at different temperatures, has compared the tension of the vapour of water in air and in a vacuum. He found that it was always feebler in the former than in the latter case; but the differences were so trifling, that the law of Gay-Lussac is not diminished in its generality and value, Applications.-In addition to the well-known application of steam, it has often been proposed to employ vapours and gases of various kinds, as a moving power; as, for instance, by the expansion of heated air, or by the alternate vaporisation and liquefaction of different substances, such as ether, carbonic acid, etc. It may be useful, however, to mention here an important principle announced for the first time in 1824, by M. S. Carnot, in a small but curious work, entitled Reflexions sur la puissance mécanique du feu, viz. that the same quantity of heat can only Fig. 191. It is composed of a glass tube A, to the extremities of which are cemented two iron stop-cocks b and d. The lower stop-produce the same amount of labour, whatever may be the cock is furnished with a short tube, which puts the tube a in nature of the gas on which it acts, provided that no loss is communication with a second tube B of smaller diameter. occasioned by improper or defective arrangements. Experience scale is placed between these two tubes, in order to measure has fully confirmed the theoretical considerations on which the height of the column of mercury contained in each. The this principle was founded by M. Carnot. In fig. 191, is repretube A is then filled with dry mercury, and the stop-cocks bsented a sort of jack or turnspit, which is very common in and d being shut, we first screw on the stop-cock 6, at the place of the funnel c, a glass globe filled with dry air or any other gas, and furnished also with a stop-cock which is closed. Next, opening the three stop-cocks, a part of the mercury is allowed to flow from the tube A, which is replaced by dry air from the globe. The stop-cocks are then closed, and as the air in the space A expands on issuing from the globe, and is under a pressure less than that of the atmosphere, it is forced back by pouring some mercury into the tube B, until the level Its mechanism is so several parts of the continent, and which is put in motion by Reflections on the Mechanical Power of Fire. |